An important component of the mammalian maternal store is ribosome. Translation of maternal mRNA in early embryo depends on the maternal ribosomes. However, a large portion of the maternal ribosomes is destroyed along with many other mRNAs during oocyte maturation. This programmed destruction of maternal ribosomes has been most perplexing and appears wasteful. From the prevailing view in ribosome biology, there is only one type of ribosome and why cannot early embryos use these ribosomes that are destroyed during oocyte maturation? Our recent results suggest an unconventional explanation to this dilemma. Based on these results, we hypothesize that oocytes transcribe several types of rRNA, some of which are used for oogenesis, whereas others for translating maternal mRNA in early embryos. We showed recently that basonuclin (BNC1, Bnc1), a cell-type specific transcription factor for the ribosomal RNA genes (rDNA), is also a mammalian maternal effect gene. Furthermore, our data suggest that BNC1 regulates a subset of rDNA. This observation led us to isolate seven subtypes of variant rDNA (v-rDNA). Subsequent characterization of these v-rDNAs showed that each variant is regulated independently and tissue-specifically. Taken together, these results suggest that perturbation of expression of a subset of rRNA in oocytes may lead to embryonic developmental failure. Our hypothesis challenges the prevailing notion that rDNA transcription units are homogeneous and regulated uniformly in all tissues. Therefore, our hypothesis, if correct, will revise current understanding of ribosomal biogenesis, especially its implication in oogenesis and early embryogenesis. To test this hypothesis, we propose two Specific Aims. Specific Aim 1 will determine if rRNA population is heterogeneous by cloning and examining the rRNA region (i.e., cytoplasmically expressed region) of each v-rDNA. Using multiple sequence alignment, this investigation will determine if variable regions (markers) exist among rRNAs, which indicate rRNA heterogeneity. Specific Aim 2 will investigate if mouse oocytes contain several different types of rRNA and whether some are selectively degraded during oocyte maturation. We will use markers identi- fied previously and in Aim 1 to examine the expression of each v-rDNA/rRNA subtypes in oo- cytes and early embryos. Using our BNC1-deficiency oocytes, we will investigate if loss of BNC1 perturbs the balance of v-rDNA expression in the maternal store. These studies will pro- vide experimental foundations for hypotheses to be tested in the ensuing R01 application. PUBLIC HEALTH RELEVANCE: Recent studies suggest that eggs of "low quality" are largely responsible for infertility in women, and yet what constitutes a "good egg"-of which the maternal store is an integral part-is unknown. The proposed studies will increase our basic understanding of what constitutes a "good egg". Specifically, we will examine if the maternal store of rRNA is heterogeneous and how this heterogeneity influences early embryonic development. Such information may ultimately translate to improving the practice of Assisted Reproductive Technology (ART).